This invention is in the technical field of magnetic resonance imaging (MRI) using nuclear magnetic resonance (NMR) phenomenon and relates to a detunable coil assembly with a simplified design and capable of detuning both quadrature modes at the same time.
It has been known for producing images by utilizing NMR to employ separate specially designed coils for producing a radio frequency (RF) excitation field and receiving the NMR signal. The excitation is commonly provided by a large-volume field coil (or a xe2x80x9cvolume coilxe2x80x9d) capable of generating a homogeneous RF magnetic field throughout a relatively large region such as around the entire body of a subject to be imaged. For receiving the NMR signals from specified localized portions of the body, smaller coils (such as so-called xe2x80x9csurface coilsxe2x80x9d which generally have excellent sensitivity and poorer B1 homogeneity) is used for detailed imaging. Since both the excitation coil and the receiving coil are tuned to resonate at the same frequency, they would interact, or couple, with each other and thereby cause non-uniformity in the excitation field.
In order to minimize such coupling, it has been known to build both the volume coil for excitation an the surface coil with an active detuning feature. While the excitation volume coil excites the sample, the surface coil is detuned to a different frequency. Immediately after the excitation phase, the volume coil is detuned to an off-resonance frequency and the surface coil is tuned back to the resonance frequency for detection. Throughout herein, therefore, xe2x80x9cdetuningxe2x80x9d will mean a frequency shift by at least the sum of the half widths of the resonance peaks of the two coils. For the purpose of these detuning operations, it has also been knownto make use of pin diodes as switches because they must be carried out within a few microseconds.
Prior art apparatus and methods for actively detuning (or xe2x80x9cdisablingxe2x80x9d) an NMR volume coil include those described in U.S. Pat. Nos. 4,833,409 (issued May 23, 1989 to M. G. Eash) and 4,763,076 (issued Aug. 9, 1988 to M. Arakawa, et al.). These prior art apparatus are characterized as having a pin diode-containing switching circuit electrically connected to the RF circuit of the field coil, and this makes it difficult to handle the currents and the problem of electrical noise becomes severe.
It is therefore an object of this invention to provide a detunable coil assembly with a simple design with which the detuning of a field coil for MRI can be accomplished.
It is another object of this invention to provide such a coil assembly with which frequencies of both quadrature modes of the field coil can be shifted at the same time.
It is still another object of this invention to provide a method of detuning an RF field coil for MRI with a detuning coil of a simple design.
It is a further object of this invention to provide a method of detuning a quadrature field coil for MRI from frequencies of both its quadrature modes.
A detunable coil assembly embodying this invention is characterized not only as comprising a main coil which resonates at a specified resonance frequency and a switchable detuning coil which, when switched on, becomes inductively coupled to the main coil so as to detune it from its resonance frequency but also wherein the detuning coil and the main coil are electrically insulated from each other. Because the main coil which serves as the field coil in an MRI application is physically separated from the low-frequency switching circuit for the detuning coil, the circuit design is much more simplified without adversely affecting the performance. The invention is applicable to situations where the main coil is a quadrature birdcage coil or a multiply tuned coil with a coil array and the main coil can be detuned from more than one of its resonance frequencies at the same time.